The present invention relates to an injection molding method and to an injection molding metal mold assembly used in such a method. More particularly, the invention relates to an insert molding method in which an injection molding machine is used to inject molding material into an insert of, for instance, aluminum foil to manufacture a molding in which the insert is integral with the molding material, and also to a metal mold assembly used in the method.
In an insert molding method of this type, insertion accuracy of the insert and the molding material is essential. For insertion by injection molding, the insert is inserted into the metal mold assembly in advance. However, this method suffers from a problem in that the insert can easily be displaced during molding, and the accuracy of the resultant molding is hence often lower than required.
Examples of the conventional method in which an insert is inserted into a metal mold assembly in advance and molding material is injected to the insert to form an insert molding are a method in which an insert plate is set along the configuration of the core mold of the metal mold assembly, a method in which an insert plate is set in the metal mold by inserting the positioning pin of the metal mold into a hole in the insert plate, and a method in which a film plate is held by a holding mold provided for the cavity mold so that the plate is automatically cut and pushed into the cavity (see Japanese Laid-Open patent application No. 5747/1981). However, in these conventional methods, the positioning accuracy of the insert is low, and the insert is not satisfactorily held. Thus, it is difficult to obtain moldings high in insertion accuracy utilizing these conventional methods.
Furthermore, the conventional methods are different from an injection molding method in which, according to the invention, displacement of the insert during the molding operation is prevented to obtain a molding high in insertion accuracy and which has a fold as the bottom or cover (top) of a can-like container.
A can-like container to which the invention relates and its bottom and cover (top) will be described.
FIG. 1 is a perspective view showing a can-like container having a side wall 1, a bottom 2 and a rectangular cover 3. FIG. 2 is also a perspective view showing a can-like container having a side wall 4, a bottom 5 and a round cover 6.
FIG. 3 is a sectional view of the bottom 2 which is to be joined to the side wall 1. In FIG. 3, reference 7 designates a base material such as metal foil or sheet which air, water, etc. cannot permeate (hereinafter referred to as "a gas barrier type base material" when applicable). Examples of the base material 7 are aluminum foil, and sheets of saponified ethylene vinyl acetate copolymer, polyvinylidene chloride and polyamide. Typically, the aluminum foil is used. A resin layer (not shown in FIG. 3) which can be made molten by heating is formed on one or both sides of the gas barrier type base material.
The molding shown in FIG. 3 is manufactured by inserting the gas barrier type base material or a base material having a thermally meltable resin layer into a metal mold where molten molding material is injected into the base material (or an insert) by an injection molding machine. In FIG. 3, reference numeral 8 designates a synthetic resin layer which is formed by injection. The molding in FIG. 3 must have a fold 9 to join it to the side wall 1.
FIG. 4 is a longitudinal sectional view of the rectangular cover 3 (FIG. 1) which is to be joined to the side wall 1. In FIG. 4, reference numeral 10 designates a base material. In the case of FIG. 4, the base material 10 is obtained by forming thermally meltable resin layers 12 and 13 and both sides of a metal foil 15. In FIG. 4, reference numeral 14 designates a synthetic resin layer. The molding in FIG. 4 can be obtained by injecting synthetic resin molding material into the base material (insert) 10, similar to the case of the bottom shown in FIG. 3. In FIG. 4, reference numeral 11 designates a handle (a so-called "pull top") on the cover. When the handle 11 is lifted with a finger tip inserted into a groove 16, the base material 10 is torn below the position 17 so that the cover 3 is pulled off. Further in FIG. 4, reference numeral 18 designates a treatment layer to facilitate the peeling of the handle portion from the base material 10. The molding in FIG. 4 has a fold 19 similar to the case of the molding in FIG. 3.
FIG. 5 is a longitudinal sectional view of the round cover 6 shown in FIG. 2. The round cover 6 is manufactured in the same manner as the molding in FIG. 4. In FIG. 5, components similar to those in FIG. 4 are designated by the same reference numerals and their descriptions are omitted. The molding in FIG. 5 also has a fold 19.
In the injection molding of a flat part which, unlike the moldings in FIGS. 3, 4 and 5, has no fold (9 or 19), for instance, in the case of injecting molding material into a round insert, a slight displacement thereof causes no trouble. On the other hand, if, in the case of a molding with a fold (which the invention concerns), the insert is displaced even slightly during molding, undesirable results occur. It is impossible to obtain a molding high in insertion accuracy.
Further, in a conventional method of adhering a multi-layer sheet to its outer synthetic resin layer to improve the gas barrier characteristics, after the outer synthetic resin layer is formed by injection molding or the like, an adhesive is used to cause the multi-layer sheet to adhere thereto.
This conventional method suffers from the following problems: (1) the number of manufacturing steps is high and the manufacturing cost high, (2) the adhesive must be hygenic, and (3) the synthetic resin layer may peel off the multi-layer sheet if repeatedly heated and cooled.
It is desirable to make the outer resin layer integral with the multi-layer sheet, for instance, by insert injection molding which prevents the permeation of moisture and oxygen and the transmission of light to improve the gas barrier characteristics, thereby to provide a can-like container's cover in which the outer resin layer is strongly joined with the multi-layer sheet. However, when the outer resin layer is joined with the multi-layer sheet, the handle (or "pull top") of the cover is also joined with the multi-layer sheet, with the result that it is impossible to peel the handle off the multi-layer sheet or the body of the cover.
Further, in order to improve the openability of the opening part, a slot or groove is, in general, formed in the cover so that the opening part is opened along the slot. However, the formation of the slot unavoidably lowers the moldability. In order to overcome this difficulty, in the formation of the resin sheet according to the aforementioned method in which the resin sheet is bonded to the aluminum foil, the slot is divided into a plurality of parts, i.e., instead of one slot, a plurality of slots connected through bridges are formed to permit the smooth flowing of the molding material to maintain the moldability (see Japanese Laid-Open patent application No. 39489/1977).
The method is advantageous in that the moldability is improved; however, it is obvious that the openability is lowered by the bridges.